1. Field of the Invention
The present invention relates to a resin composition and its application. Specifically, the present invention relates to an epoxy resin composition having at least two different styrene-maleic anhydride (SMA) copolymers with different molar ratios of styrene to maleic anhydride as a hardener, and a prepreg and a laminate prepared by using the epoxy resin composition.
2. Descriptions of the Related Art
Printed circuit boards are circuit substrates that are used for electronic devices to load other electronic components and to electrically connect the components so as to provide a stable circuit working environment. One kind of conventional printed circuit boards is a copper clad laminate (CCL), which is primarily composed of resin, reinforcing material and copper foil. The resin may be, for example, epoxy resin, novolac resin, polyamine formaldehyde resin, silicone resin or polytetrafluoroethylene resin; and the reinforcing material may be, for example, glass fiber cloth, glass fiber mat, insulating paper or linen cloth.
Considering the subsequent electronic processes, the properties such as heat resistance, size stability, chemical stability, workability, toughness, and mechanical strength etc. should be taken into consideration during the preparation of the printed circuit board. Generally, epoxy resin is the most popular resin in the printed circuit board industry since a printed circuit broad prepared by using epoxy resin can properly meet the above requirements. Epoxy resin is generally considered as an organic, high molecular compound comprising two or more epoxy groups and is a reactive monomer. A highly cross-linked structure can be obtained by polymerizing molecules with a high epoxy group content. Although a highly cross-linked structure possesses a relatively high hardness and glass transition temperature (Tg) as well as a good chemical resistance, it also possesses poor impact resistance which is disadvantageous to the subsequent processes.
FR-4 laminate prepared by using epoxy resin, for example, has a relatively high dielectric constant (Dk) and dissipation factor (Df). A high Dk will slow down the signal transmission rate of the laminate and a high Df will make part of the signals produce energy transformation and then be lost within the laminate due to material resistance. Therefore, the industry is committed to improve and provide a lower Dk and Df.
Because styrene-maleic anhydride copolymer (hereinafter abbreviated as SMA copolymer; see formula (I) as below) has a capability of lowering Dk and Df, it is commonly used as an epoxy resin hardener. For each SMA copolymer unit, the higher the molar ratio of styrene to maleic anhydride (i.e. greater m value) the lower the Df value of the prepared laminate and the better the electrical properties of the prepared laminate, while the Tg value of the laminate is relatively low (e.g., m=1, Tg is about 155° C.; m=8, Tg is usually about 104° C.). In addition, if m is too high (e.g. ≧8), the surface stickiness of the prepregs for preparing laminates will be higher and the prepregs will tend to stick with each other, which is unfavorable to the operation. On the contrary, the lower the molar ratio of styrene to maleic anhydride in each SMA copolymer unit (eg. m≦3) the higher the Tg of the prepared laminate and the better the heat resistance of the prepared laminate, but the laminate will become brittle, which will result in powder pollution during the processing.

WO9818845 disclosed a method to improve the brittleness, which uses tetrabromobisphenol A (TBBPA or TBBA), tetrabromobisphenol A diglycidyl ether (TBBAPDGE), or a mixture thereof as a co-hardener and uses SMA copolymer as a hardener to cure the FR-4 epoxy resin so as to increase the toughness, Tg, and stability of the laminate. Furthermore, EP417837 and WO9607683 respectively described an allyl-containing (for example, triallyl isocyanurate (TAIC)) IPN polymer resin composition for printed circuit boards. Although the addition of TAIC can improve the toughness of the prepared laminate, it is unfavorable to the electrical property (high Df) and the promotion of Tg achieved thereby is not significant.
In view of the above, the present invention provides a novel epoxy resin composition. The surface of the prepregs prepared by using the epoxy resin composition will not stick to each other and the laminate prepared by these prepregs is provided with good wet fastness and solder floating resistance as well as good electrical properties (low Dk/Df) and toughness.